nected with the cushion, and two insulated pith-balls be electrified by it, they will repel each other.

(3) If one insulated ball be electrified by the prime conductor, and another by the conductor connected with the cushion, they will attract each other.

(4.) If one ball be electrified by glass, and another by wax, they will attract each other.

(5.) If one ball be electrified by a smooth, and another by a rough excited glass tube, they will attract one another.

CONVERSATION XXXII.

Of Electrical Attraction and Repulsion.

TUTOR. I will show you another instance or two of the effects of electrical attraction and repulsion.

This apparatus (Plate v11. Fig. 4.) consists of three bells suspended from a brass wire, the two outer ones by small brass chains; the middle bell, and the two clappers xx, are suspended on silk. From the middle bell there is a chain N, which goes to the table, or any other conducting substance. The bells are now to be hung by c on the conductor, and the electrical machine to be put in motion.

James. The clappers go from bell to bell, and make very pretty music: how do you explain this?

Tutor. The electric fluid runs down the chains a and b to the bells A B, these having more than their natural quanticy, attract the clapper xx which take a portion from A and B, and carry it to the centre bell N, and this, by means of the chain, conveys it to the earth.

Charles. Would not the same effect be produced if the clappers were not suspended on silk?

Tutor. Certainly not: nor will it be produced if the chain be taken away from the bell N, because then there is no way left to carry off the electric fluid to the earth.

Another amusing experiment is thus shown. Let there be two wires placed exactly one above another, and parallel; the upper one must be suspended from the conductor, the other is to communicate with the table. A light image placed between these will, when the conductor is electrified, appear like a rope-dancer.

This piece of leaf brass is called the elec tric fish, one end is a sort of obtuse angle, the other is acute: if the large end be presented towards an electrified conductor, it will fix to it, and, from its wavering motion, it will appear to be animated.

This property of attaction and repulsion has led to many inventions of instruments called electrometers.

James. Is not an electrometer a machine to measure the strength of the electricity?

Tutor. Yes; and this is one of the most simple (Plate VII. Fig. 5.,) and it depends entirely upon the repulsion which takes place between two bodies in a state of electrification. It consists of a light rod and a pithball, hanging parallel to the stem, but turning on the centre of a semicircle, so as to keep close to its graduated limb. This is to be placed in a hole a on the conductor L, and according as the conductor is more or less electrified, the ball will fly farther from the stem.

Charles. If the circular part be marked with degrees, you may ascertain, I suppose,

pretty accurately, the strength of any given charge.

Tutor. Yes, you may; but you see how fast the air carries away the electricity, it scarcely remains a single moment in the place to which it was repelled. Two pith balls may be suspended parallel to one another, on silken threads, and applied to any part of an electrical machine, and they will by their repulsion, serve for an electrometer, for they will repel one another the more, as the machine acts more powerfully.

James. Has this any advantage over the other?

Tutor. It serves to show whether the electricity be negative or positive; for if it be positive, by applying an excited stick of sealing-wax, the threads will fall together again; but if it be negative, excited sealingwax, or resin, or sulphur, or even a rod of glass, the polish of which is taken off, will make them recede farther.

We have now perhaps said enough respecting electrical attraction and repulsion, at least for the present; I wish

you,

how